views: refactor atom feed links.

* Fix comment & forum feeds to include search params.
* Remove global post feeds (only include post feeds on post index).

app/helpers/application_helper.rb

@@ -297,6 +297,10 @@ module ApplicationHelper
     end
   end
 
+  def atom_feed_tag(title, url = {})
+    content_for(:html_header, auto_discovery_link_tag(:atom, url, title: title))
+  end
+
   def show_moderation_notice?
     CurrentUser.can_approve_posts? && (cookies[:moderated].blank? || Time.at(cookies[:moderated].to_i) < 72.hours.ago)
   end